Communication networks consist of interconnected nodes and can be subdivided into core networks and access networks, the latter providing access to user equipment, for example a wireless access for mobile user equipment to a radio access network. Core networks interconnect access networks and further networks, e.g. other core networks or the Internet. In the UMTS architecture, an access network can be controlled by an RNC (radio network controller) which is connected to the core network and provides access to the core network, i.e. serves as access node.
For the transmission on a connection, speech is encoded according to a coding scheme, alternatively denoted as codec. A coding scheme can transport speech either in a compressed or in a non-compressed mode. In many networks, different coding schemes can be used and different nodes can have different capabilities for handling the coding schemes. Transcoders perform the transcoding and rate adaptations between different coding schemes. TFO (Tandem Free Operation) is a configuration of two transcoders with compatible coding schemes on the compressed voice sides at both ends of a connection, i.e. on the interface to the user equipment. In this case, the transcoding stages can be bypassed and the compressed voice coding used end to end in the connection.
Out of band coding negotiation allows that speech connections are established end to end with a common coding scheme, i.e. the same speech coding is used in the whole connection between the access networks. The advantage is that maintaining compressed voice saves core network bandwidth and optimizes speech quality because transcoding stages are avoided.
The ITU (International Telecommunication Union) protocol BICC (bearer independent call control) supports out of band signaling procedures which allow a negotiation of the coding scheme between network nodes. In the ITU-T proposal BICC Q.1901 (International Telecommunication Union, June 2000), coding scheme negotiation is performed from the originating control node in a connection to each subsequent node by including a list of allowed coding schemes in the APP (Application Transport Parameter) parameter in the IAM (Initial Address Message) for the set-up of the connection. Each node checks the list and if it does not support a particular coding type it removes it from the list. The adapted list is passed on with the IAM and any non-supported types are removed as long as the BICC signaling is supported. When the final node, either the terminating node or the last node supporting BICC is reached, the coding scheme type with highest priority is selected. This coding scheme and the list (reduced to an active coding scheme list) are returned to the originating node via all intermediate nodes.
If a transcoder selection is performed by a node at a PLMN (Public Land Mobile Network) border then the indication of the selected coding scheme to subsequent nodes in the section of the connection through the PLM network is a compressed voice coding scheme. In the BICC coding scheme negotiation procedures there are no rules defining how many transcoder stages are allowed and whether an access network that supports out of band coding scheme negotiation can insert transcoders to keep TrFO (transcoder free operation) between the access node and the rest of the network. The number of transcoding stages in a connection end to end can seriously affect the speech quality. If TFO is not possible, more than three transcoding stages typically cause substantial speech degradation. The number of stages causing a substantial degradation depends on the coding algorithm and the speech degradation by further entities in the connection.
The coding scheme negotiation procedures may result in transcoders being inserted to enable supplementary services or because the bearer technology in a node or network does not support compressed voice. For example, ATM (Asynchronous Transfer Mode) networks allow the transmission of either compressed or non-compressed speech while STM (Synchronous Transfer Mode) networks require non-compressed speech coding. Furthermore, the negotiation should result in the optimum location of the transcoders which is not always the case. For example, for connections exiting an STM network to ATM, a transcoder should be located at the network edge to save bandwidth in the ATM network by use of a compressed coding scheme.
In many cases, it is necessary to modify the coding scheme in a section of a connection. For example, a connection is often transferred between different access networks due to a handover. Modifications in the core network are disadvantageous, especially if they require increased transmission bandwidth which will sometimes not be available causing a termination of the connection. The transcoder number in a connection can be increased by a modification with corresponding quality degradation. Again, an optimum location of transcoders is often not achieved.